This invention generally relates to bypass type flow control of a fixed displacement pump used in a load responsive system.
In more particular aspects this invention relates to unloading controls of a pump bypass type control for use in load responsive systems.
In still more particular aspects this invention relates to unloading controls for load responsive bypass type pump controls, which reduce the pump discharge pressure below the level of the control pressure differential, when the control system is maintained in a standby condition.
Load responsive controls of pump output flow are widely used and are very desirable in load responsive systems, since they improve the system efficiency and the control characteristics of the system valves. Such a load responsive pump control of a bypass type is shown in U.S. Pat. No. 3,488,953, issued to Haussler. When using such a control the minimum pump discharge pressure, even in standby condition, is dictated by the value of the control pressure differential, which may be in the order of say 200 PSI and especially with large pumps represents a high energy loss, with the system in standby condition.
The efficiency of such a control can be increased by providing an unloading control, which permits reduction in the pump discharge pressure in the standby condition to a level, below that equivalent to the control pressure differential. Such a control is shown in my U.S. Pat. No. 3,882,896, issued May 13, 1975 and also in my U.S. Pat. No. 4,159,724, issued Jul. 3, 1979. Those unloading controls although very effective suffer from one basic disadvantage. Since deactivation of the unloading control is accomplished by the load pressure signal and since the system may be controlling very small loads, the pump discharge pressure, in its standby condition, can only be reduced to a pressure level, which will provide sufficient energy for deactivation of the unloading control. This minimum pressure level, although lower than the control pressure differential, must still be of sufficient magnitude, so that it still represents a very significant loss in standby condition. Also there is an additional disadvantage to this approach and that is the comparatively slow response of the unloading control, in activation of the load responsive control of the pump. This feature becomes especially important when the controlled load may reverse direction and therefore from being of positive type becomes of a negative type. Under these conditions, during negative load control, the unloading control becomes activated, adversely affecting the response characteristics of the control system operating such a load.